fix: ignore SSE retry: field to prevent McpTransportException

The SSE specification includes 'retry:' as a valid field for setting
the reconnection interval. However, the SseLineSubscriber treats any
unrecognized line as an error, causing McpTransportException when
MCP servers (e.g., rmcp-based servers like SurrealDB) emit retry
directives in SSE responses.

This fix adds a handler that silently ignores 'retry:' lines, matching
the existing behaviour for ':' comment lines.

Related: servers using rmcp (Rust MCP SDK) send 'retry: 3000' by
default, which triggers this error and breaks Streamable HTTP
connections to SurrealDB and other rmcp-based MCP servers.

Author: 邵广超

mcp-core/src/main/java/io/modelcontextprotocol/client/transport/ResponseSubscribers.java

@@ -179,6 +179,11 @@ else if (line.startsWith(":")) {
 					logger.debug("Ignoring comment line: {}", line);
 					upstream().request(1);
 				}
+				else if (line.startsWith("retry:")) {
+					// Ignore SSE retry directive — only the server controls reconnection
+					logger.debug("Ignoring retry directive: {}", line);
+					upstream().request(1);
+				}
 				else {
 					// If the response is not successful, emit an error
 					this.sink.error(new McpTransportException(